Williams also quoted Martin Luther King Jr., saying "there comes a time when silence is betrayal"

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Serena Williams reacts after losing a point to Karolina Pliskova, of the Czech Republic, during the semifinals of the U.S. Open tennis tournament, Thursday, Sept. 8, 2016, in New York. The tennis star vowed she "won't be silent" about police killings.

Star tennis player Serena Williams says she "won't be silent" about the killing of black men by police officers.

Williams wrote on Facebook on Tuesday that she was inspired to speak out after asking her black 18-year-old nephew to drive her to a meeting. Their car traveled past a police officer, and Williams "quickly checked to see if he was obliging by the speed limit" so as to avoid an interaction with the officer.

Williams recalled "that horrible video of the woman in the car when a cop shot her boyfriend" — a reference to when Philando Castile was shot by a Minnesota police officer in July — and said she worried about similar fates for members of her family.

Williams joins a high-profile group of athletes speaking out about social injustice. San Francisco 49ers backup quarterback Colin Kaepernick began kneeling during the national anthem in the preseason and has been joined by professional, collegiate and high school athletes nationwide.